Many bacteria produce antibacterial peptides or proteins (e.g., bacteriocins) that are generally active against other bacteria, typically closely related. An exemplary list of bacteria and their bacteriocins are shown in Table 1.
The classical bacteriocins are the colicins produced by Escherichia coli. Most colicins are relatively large proteinaceous compounds that are not actively secreted from the bacterial cell. Microcins produced by E. coli are peptides or polypeptides that are secreted from the cell by a dedicated export pathway and are post-translationally modified (Class I microcins) or are not posttranslationally modified (Class II microcins). Posttranslational modification requires the production of enzymes that modify the ribosomally translated peptide.
Bacteriocins produced by LAB are normally active against other Gram-positive bacteria, especially closely-related LABs. Likewise, bacteriocins produced by Gram-negative bacteria are against Gram-negative target strains. For example, colicin V, a bacteriocin produced by Escherichia coli, is active against a wide range of other E. coli. 
Colicin V was the first colicin discovered from E. coli. It is a Class II microcin that is synthesized as a 105 amino acid pre-peptide (leader+bacteriocin) that is cleaved to release the active 88 amino acid mature peptide. The colicin V operon includes a structural gene, an immunity gene, and two dedicated transport genes.
A large number of LAB produce bacteriocins that include the lantibiotic peptides (Class I); non-lantibiotic peptides (Class II); and proteins (Class III). The lantibiotics, e.g., nisin produced by Lactococcus lactis subsp. lactis, are post-translationally modified and have a genetic operon consisting of about 11 genes for their synthesis, immunity, modification and export from the cell. The non-lantibiotic (Class II) bacteriocins are similar to colicin V in genetic complexity. These bacteriocins are produced as pre-peptides that are cleaved to form the mature peptide and exported from the cell in the same way as colicin V, e.g. carnobacteriocins A and B2, leucocin A, and pediocin PA-1. The non-lantibiotic divergicin A produced by Carnobacterium divergens UAL9 requires only two genes for its production and secretion from the cell. Secretion is under the control of the cell's general secretory (sec) pathway. Predivergicin A consists of a signal peptide and divergicin A. One gene or nucleotide sequence encodes a bacteriocin. The other gene encodes an immunity protein.
To date no bacteriocins produced by LAB have been discovered that are active against Gram-negative bacteria, such as E. coli. For reasons that will become more evident below, it may be desirable to select a Gram-positive host that produces a bacteriocin active against one or more gram-negative bacteria. For example, LAB could target E. coli if it is genetically modified (GMO) to produce a bacteriocin (such as, colicin V) or another bacteriocin that is active against another target bacterium.
Further, the ability to target a Gram-negative bacterium, such as E. coli, using a Gram-positive bacterium that expresses a bacteriocin effective against the Gram-negative bacterium, suggests the possibility of an alternative or supplemental therapy or preventative treatment protocol against any diseases or conditions caused by the Gram-negative bacteria. An example of such a condition is post-weaning diarrhea (PWD), also known as scours, which is caused by an E. coli infection in pigs.
Outbreaks of E. coli PWD or scours are an ongoing problem in pig production. PWD or scours typically result in significant weight loss of the affected animals.
A need exists for treatments that promote weight gain or, at a minimum, result in no further weight loss during infection.